Groundwater flow through Pleistocene glacial deposits in the rapidly urbanizing Rouge River–Highland Creek watershed,City of Scarborough,southern Ontario,Canada

The city of Scarborough lies on the eastern margin of the Greater Toronto Area of southern Ontario, Canada, along the northern coastline of Lake Ontario. The City has a population of 500,000 and is presently one of the fastest growing communities in Canada. The City is expanding northwards onto rural land on the south slope of the large Pleistocene glacial Oak Ridges Moraine system. The moraine system is underlain by a thick (150 m) succession of tills, sands and gravels and is a regionally-significant recharge area for three principle aquifer systems that discharge to numerous watercourses that flow to Lake Ontario. Protection of deeper aquifers from surface-generated urban contaminants is a particular concern. A groundwater flow model using Visual MODFLOW was developed for the 350-km² Rouge River–Highland Creek (RRHC) drainage basin using an extensive GIS-based collection of subsurface geological, geophysical and hydrogeological data, maps of land use and surficial geology. The RRHC model was calibrated against point water level data, known potentiometric surfaces of the principal aquifers and baseflow measurements from streamflow gauging stations and determined to be within acceptable limits. Water balance calculations indicate that 70% of the basin recharge (106,000 m³/day) enters the Upper Aquifer along the crest and immediate flanks of the Oak Ridges Moraine. To the south, Upper Aquifer water moving through fractured till aquitards accounts for more than 75% of recharge to deeper aquifers. Water quality data confirm previous observations that urban- and rural-sourced contaminants (chlorides and nitrates) present in Upper Aquifer waters are moving rapidly into deeper aquifers. Some 83% of total RRHC recharge water is ultimately discharged as baseflow to creeks draining to Lake Ontario; the remainder discharges to springs and along eroding lakeshore bluffs. Model results demonstrate that deeper aquifers are poorly protected from urban contaminants and that long-term protection of ground and surface water quality has to be a priority of municipal planners if the resource is not to be severely degraded. Electronic Publication     

Deconvolving the pre-Himalayan Indian margin-Tales of crustal growth and destruction

The metamorphic core of the Himalaya is composed of Indian cratonic rocks with two distinct crustal affinities that are defined by radiogenic isotopic geochemistry and detrital zircon age spectra. One is derived predominantly from the Paleoproterozoic and Archean rocks of the Indian cratonic interior and is either represented as metamorphosed sedimentary rocks of the Lesser Himalayan Sequence(LHS) or as slices of the distal cratonic margin. The other is the Greater Himalayan Sequence(GHS) whose provenance is less clear and has an enigmatic affinity. Here we present new detrital zircon Hf analyses from LHS and GHS samples spanning over 1000 km along the orogen that respectively show a striking similarity in age spectra and Hf isotope ratios. Within the GHS, the zircon age populations at 2800-2500 Ma,1800 Ma, 1000 Ma and 500 Ma can be ascribed to various Gondwanan source regions; however, a pervasive and dominant Tonianage population(～860-800 Ma) with a variably enriched radiogenic Hf isotope signature(eHf = 10 to-20) has not been identified from Gondwana or peripheral accreted terranes. We suggest this detrital zircon age population was derived from a crustal province that was subsequently removed by tectonic erosion. Substantial geologic evidence exists from previous studies across the Himalaya supporting the Cambro-Ordovician Kurgiakh Orogeny. We propose the tectonic removal of Tonian lithosphere occurred prior to or during this Cambro-Ordovician episode of orogenesis in a similar scenario as is seen in the modern Andean and Indonesian orogenies, wherein tectonic processes have removed significant portions of the continental lithosphere in a relatively short amount of time. This model described herein of the pre-Himalayan northern margin of Greater India highlights the paucity of the geologic record associated with the growth of continental crust. Although the continental crust is the archive of Earth history, it is vital to recognize the ways in which preservation bias and destruction of continental crust informs geologic models.     

Assessing the significance of wet-canopy evaporation from forests during extreme rainfall events for flood mitigation in mountainous regions of the United Kingdom

There is increased interest in the potential of tree planting to help mitigate flooding using nature-based solutions or natural flood management. However, many publications based upon catchment studies conclude that, as flood magnitude increases, benefit from forest cover declines and is insignificant for extreme flood events. These conclusions conflict with estimates of evaporation loss from forest plot observations of gross rainfall, through fall and stem flow. This study explores data from existing studies to assess the magnitudes of evaporation and attempts to identify the meteorological conditions under which they would be supported. This is achieved using rainfall event data collated from publications and data archives from studies undertaken in temperate environments around the world. The meteorological conditions required to drive the observed evaporation losses are explored theoretically using the Penman–Monteith equation. The results of this theoretical analysis are compared with the prevailing meteorological conditions during large and extreme rainfall events in mountainous regions of the United Kingdom to assess the likely significance of wet canopy evaporation loss. The collated dataset showed that event Ewc losses between approximately 2 and 38% of gross rainfall (1.5 to 39.4 mm day⁻¹) have been observed during large rainfall events (up to 118 mm day⁻¹) and that there are few data for extreme events (>150 mm day⁻¹). Event data greater than 150 mm (reported separately) included similarly high percentage evaporation losses. Theoretical estimates of wet-canopy evaporation indicated that, to reproduce the losses towards the high end of these observations, relative humidity and the aerodynamic resistance for vapour transport needed to be lower than approximately 97.5% and 0.5 to 2 s m⁻¹ respectively. Surface meteorological data during large and extreme rainfall events in the United Kingdom suggest that conditions favourable for high wet-canopy evaporation are not uncommon and indicate that significant evaporation losses during large and extreme events are possible but not for all events and not at all locations. Thus the disparity with the results from catchment studies remains.     

Contrasting flow pathways within tropical forest slopes of Ultisol soils

There are very few experimental studies identifying hydrological pathways within rain forest slopes. Such knowledge is, however, necessary to understand why forest disturbance affects rainfall–riverflow response and nutrient migration. This study examines flow pathways within lowland rain forest slopes comprising Udults of the Ultisol soil order. Experimentation was conducted on four SE Asian hillslope units (each 5 × 5 m in plan) in the Bukit Timah catchment (Singapore Island), and in the W8S5 catchment (Sabah, Borneo Island). The flow pathways were identified by artificial tracer experiments. We evaluated how well hydrometric calculations based on tensiometry and permeametry measurements predicted the tracer patterns. The tracer work indicated much faster subsurface flows at Bukit Timah than W8S5 for the storms studied. Some explanation of the greater subsurface waterflows at Bukit Timah in comparison to W8S5 is afforded by the less steep moisture release curves which maintain hydraulic conductivity as the soil dries. Vertical flow of the tracer through the upper 1 m of soil predominated (>90 per cent of percolation) in the Bukit Timah slopes. In some contrast, a major component (approximately 60 per cent) of the tracer percolation was directed laterally within the W8S5 slopes. The flow vectors calculated using the hydrometric methods did, however, grossly under‐estimate the degree of lateral deflection of waterflow generated at W8S5 and to a lesser extent over‐estimated it at Bukit Timah. In part, these errors may relate to the inability of traditional hydrometric techniques to fully characterize the effect of the large and small ‘natural soil pipes’ present within both catchments. In conclusion, the study indicates that marked variations in flow vectors exist within the Udult great group of SE Asian soils and hydrometric calculations may be poor predictors of these dominant pathways. Copyright © 2005 John Wiley & Sons, Ltd.     

On Growth and Form in Geomorphology

Perhaps surprisingly, geomorphology's relative failure to deliver meaningful process-based accounts of landscape development has not stimulated much in the way of procedural debate. Although most geomorphologists seem to agree that a problem exists — how best to make explicit the links between process and form? — this tends to be seen as a substantive problem only, the solution to which lies within the existing framework of geomorphic research, located broadly within the tradition of positivist scientific method. Here I argue that we need to ask a new type of question in a new way: one which gives priority to organizational/compositional relationships rather than to detailed process studies, within the revived context of space–time dynamics. Such a framework draws loosely on complexity theory and realist philosophy, and, in the first instance at least, suggests a return to conceptual, qualitative methods of research. © 1997 by John Wiley & Sons, Ltd.     

The inner city: In search of the problem

This paper explores the theoretical content of the inner-city debate in Britain and seeks to examine the extent to which the ‘problem’ is either new or primarily economic. It is argued that the inner-city issue, while subject to specific political and ideological forces, is associated with the general processes of uneven capitalist development. In this sense, metropolitan decline is a further manifestation of the relationship between the built environment, the transience of economic activity and social change. More specifically the decline of the inner city relates to the incipient role of Britain in the development of capitalism and its current position in the world economy. The emergence of this as a ‘problem’ for the British state is combined with a convenient spatial determinism, which isolates the inner city as an anomaly even as the search for palliatives is erected to the status of a major social task.     

Retrieval of atmospheric reflectivity profiles in case of long radar pulses

Future space-borne cloud profiling radars (CPR) will employ longer pulse lengths than ground-based systems. As a consequence, reflectivity profiles observed from space will be more strongly convoluted with the radar pulse shape. Here we describe a constrained linear inverse technique that reduces the effects of this convolution. We apply the technique to simulated data, based on actual observations from the ground, and show that significant improvements in the reflectivity profile can be obtained. Average Z errors (dBz) are halved, while cloud boundaries are substantially better retrieved. The results in this paper are relevant to space-borne missions like CloudSat and EarthCARE.